RF switch modules are a key building block in front end of wireless systems and find many uses in applications such as mobile phones and wireless local area networks (LANs). Performance metrics such as low insertion loss, high linearity, high isolation, high power handling and switching time are critical in RF switch module design. Switch modules target at LTE cellular applications include a duplexer and a radio frequency switch integrated circuit (IC).
An RF switch may comprise multiple switching elements which control the flow of RF power between different points in a particular application. Switching elements are comprised of transistors that are stacked. The number of transistors in a stack is determined by the maximum RF voltage that can be developed across the complete stack and the maximum voltage that can be tolerated across an individual transistor without introducing limitations to either reliability or linearity performance of device. RF switch modules are subject to stringent linearity specifications which require tight limits on the level of distortion that a device can introduce on an RF signal. Linearity requirements may be expressed by specifying different parameters such as harmonics generation, intermodulation distortion (IMD2, IMD3) or intercept points, (IP2, IP3). Irrespective of how requirements are specified the linearity performance is determined by distortion sources within the RF transistor stacks and the interaction of these with impedances external to RF switch IC on the module. Adoption of 4G/LTE standards places an increasing demand on linearity requirements for the RF switch module. Carrier aggregation and diversity implementations mean that there may be multiple frequency bands simultaneously active leading to tighter linearity constraints where harmonics from one frequency band fall within another frequency band. Centre frequency for transmission of LTE Band 17 is 710 MHz. The 3rd harmonic from this, 2.13 GHz, falls within the receive frequency range for LTE Bands 1 and 4. The harmonic distortion that can be introduced on Band 17 path by the RF switch module is required to be significantly less than previously required in other frequency bands. Existing design approaches cannot meet these linearity requirements without compromising on other performance parameters, particularly insertion loss.
There is therefore a need to provide an RF switching system which addresses at least some of the drawbacks of the prior art.